In recent years, with increase in demand of communication, a small-diameter optical fiber cable in which optical fibers are mounted in a high density has been required. Examples of such an optical fiber cable include an optical fiber cable having a structure using no slot, such as an optical fiber cable having a core obtained by assembling optical fiber core wires (or units each of which is a bundle of optical fiber core wires) (for example, refer to Patent Document 1).
In an optical fiber cable, a strain characteristic of an optical fiber at a position far away from a bending neutral line is important when bending is applied thereto. Therefore, in order to reduce the strain amount, an optical fiber core wire (or a unit) is generally twisted in one direction or an SZ direction. Particularly, an optical fiber core wire is easily branched, and therefore an SZ-twisted optical fiber cable is requested frequently.
Examples of a structure for holding a twisted state of an optical fiber core wire (or a unit) in an SZ-twisted optical fiber cable include (i) winding of a fibrous interposition and (ii) restriction by a cable sheath.
When winding of an interposition is employed, cost is high due to use of the interposition. In addition, it is necessary to cut the interposition for taking out the optical fiber core wire, and therefore work time is increased. Furthermore, the optical fiber core wire may be cut by accident.
Examples of the optical fiber cable employing the structure for holding a twisted state of a unit by a sheath include an optical fiber cable 110 shown in FIGS. 8(A), 8(B), 9(A), and 9(B).
As shown in FIG. 8(A), the optical fiber cable 110 includes a core 13, a pair of tension members 4, a pair of rip cords 5, and a sheath 6.
The core 13 is constituted by assembling a plurality of units 32 each including a plurality of optical fibers 1. The units 32 are disposed by twisting the units 32 doubly so as to form two layers (inner layer 21 and outer layer 22).
As shown in FIGS. 9(A) and 9(B), each of the units 32 constituting the inner layer 21 and the outer layer 22 has an SZ-twisted shape.
As shown in FIG. 8(A), the pair of tension members 4 is disposed so as to face each other with the core 13 interposed therebetween.
The sheath 6 covers the core 13, the rip cords 5, and the tension members 4 collectively. The sheath 6 holds a twisted state of the units 32.
As shown in FIG. 8(A), a force to remove twisting (force in an untwisting direction indicated by a void arrow) occurs in each of the units 32 by rigidity (or tension).
The sheath 6 is softened immediately after being formed, and therefore is easily deformed by the force in an untwisting direction. Therefore, the force in an untwisting direction (void arrow) may become larger than a holding force by the sheath 6 (black arrow), and a cross-sectional shape of the optical fiber cable 110 may be deformed to become noncircular as shown in FIG. 8(B).
When the cross-sectional shape of the optical fiber cable 110 becomes noncircular, disadvantages may occur in sealing of water by a closure or the like, wiring into a conduit, and the like.